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3.4 Expression Wrapper: #%expression


(#%expression expr)

Produces the same result as expr. Using #%expression forces the parsing of a form as an expression.

> (#%expression (+ 1 2))


> (#%expression (define x 10))

eval:8:0: define: not allowed in an expression context

  in: (define x 10)

The #%expression form is helpful in recursive definition contexts where expanding a subsequent definition can provide compile-time information for the current expression. For example, consider a define-sym-case macro that simply records some symbols at compile-time in a given identifier.
(define-syntax (define-sym-case stx)
  (syntax-case stx ()
    [(_ id sym ...)
     (andmap identifier? (syntax->list #'(sym ...)))
     #'(define-syntax id
         '(sym ...))]))
and then a variant of case that checks to make sure the symbols used in the expression match those given in the earlier definition:
(define-syntax (sym-case stx)
  (syntax-case stx ()
    [(_ id val-expr [(sym) expr] ...)
     (let ()
       (define expected-ids
          (λ ()
             "expected an identifier bound via define-sym-case"
       (define actual-ids (syntax->datum #'(sym ...)))
       (unless (equal? expected-ids actual-ids)
          (format "expected the symbols ~s"
       #'(case val-expr [(sym) expr] ...))]))

If the definition follows the use like this, then the define-sym-case macro does not have a chance to bind id and the sym-case macro signals an error:
> (let ()
    (sym-case land-creatures 'bear
              [(bear) 1]
              [(fox) 2])
    (define-sym-case land-creatures bear fox))

eval:11:0: sym-case: expected an identifier bound via


  at: land-creatures

  in: (sym-case land-creatures (quote bear) ((bear) 1)

((fox) 2))

But if the sym-case is wrapped in an #%expression, then the expander does not need to expand it to know it is an expression and it moves on to the define-sym-case expression.
> (let ()
    (#%expression (sym-case sea-creatures 'whale
                            [(whale) 1]
                            [(squid) 2]))
    (define-sym-case sea-creatures whale squid)


Of course, a macro like sym-case should not require its clients to add #%expression; instead it should check the basic shape of its arguments and then expand to #%expression wrapped around a helper macro that calls syntax-local-value and finishes the expansion.